Electron beam-induced etching

ABSTRACT

Beam-induced etching uses a work piece maintained at a temperature near the boiling point of a precursor material, but the temperature is sufficiently high to desorb reaction byproducts. In one embodiment, NF3 is used as a precursor gas for electron-beam induced etching of silicon at a temperature below room temperature.

This application is a continuation of U.S. application Ser. No.13/914,312, filed Jun. 10, 2013, which is hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to charged particle beam processing, andin particular to beam-induced chemical processes.

BACKGROUND OF THE INVENTION

“Beam chemistry” referred to chemical reactions initiated by a beam,such as a charged particle beam or a laser beam. “Electron beamchemistry” includes electron beam-induced deposition (EBID) and electronbeam-induced etching (EBIE) and is typically performed in a scanningelectron microscope (SEM). In both these electron beam processes,molecules of a precursor gas are adsorbed onto a work piece surface. Anelectron beam is directed at the work piece, and the electronsdissociate the adsorbates, generating reaction products. In EBID,non-volatile reaction products remain on the substrate surface as adeposit, while volatile reaction products desorb. In EBIE, one or moreof the precursor molecule decomposition products react with the workpiece surface, generating volatile reaction products that desorb fromthe work piece, removing surface material. Similar processes occur inion beam-induced deposition (IBID) and ion beam-induced etching (IBIE),although the much greater mass of the ions also causes material to beremoved from the substrate by sputtering, that is, by momentum transferfrom the energetic ions, without any chemical reaction. The mechanism bywhich the ion beam interacts with the adsorbate is thought to bedifferent from the mechanism by which the electron beam reacts with theadsorbate.

To be useful as a precursor gas the molecules need to have very specificproperties: they need to stick to the surface for a sufficient time tobe dissociated, but must not form a thick layer that shields the surfacefrom the beam, and they should not react spontaneously with the workpiece surface material in the absence of the beam. In the case ofetching, the precursor dissociation products should form a volatilecompound with the substrate material and in the case of deposition, theprecursor should decompose in the presence of the beam to deposit thedesired material. Other reaction products should be volatile so thatthey do not remain on the surface and can be removed from the system bya vacuum pump.

Beam chemistry driven by a charged particle beam is typically performedin a vacuum chamber using a gas injection system having a capillaryneedle that directs gas toward the impact point of the beam. The gasexpands rapidly and while the local gas pressure at the surface issufficient to support beam-induced reactions, the pressure in the restof the sample chamber is sufficiently low that secondary electrons canbe detected using a conventional detector, such as thescintillator-photomultiplier combination commonly referred to as anEverhart-Thornley detector.

Electron beam chemistry can also be performed with a work piece in anenvironment flooded with the precursor gas, with most of the beam pathseparated from the gaseous environment by a pressure-limiting aperture.Because the gas pressure does not permit conventional secondary electrondetection for imaging, imaging can be performed using gas cascadeamplification in which secondary electrons from the sample areaccelerated and ionize gas molecules. Electrons from the ionized gasmolecules are accelerated and ionize other gas molecules, in a cascadethat greatly amplifies the original secondary electron signal. A systemin which the sample is maintained in a gaseous environment is typicallycalled an environmental scanning electron microscope or a high-pressurescanning electron microscope (HPSEM). Gases that are not readily ionizedare not useful for forming an image using gas cascade amplification.Gases that are susceptible to dielectric breakdown are also not usefulfor forming an image using gas cascade amplification.

XeF₂ has, to date, been the most commonly used precursor gas forbeam-induced etching. However, XeF₂ has some undesirable effects. XeF₂spontaneously etches many materials, including silicon and TaN. XeF₂ isnot an optimal HPSEM imaging medium in that it provides poor chargestabilization and poor image quality during EBIE processing. XeF₂ ishighly corrosive and toxic. XeF₂ cannot be mixed with many common gasesused for residual carbon removal and surface species control. Moreover,large quantities of XeF₂ cause instability in some differentially pumpedelectron beam systems because of poor ion getter pumping of xenon.

An electron beam can also be used in electron beam lithography. Theelectron beam exposes photoresist as the beam scans in a pattern. Eitherexposed areas or unexposed areas, depending on the properties of theresist, are then removed, leaving a patterned photoresist layer. Ice canbe used as a patterning material, with the electron beam causing the iceto sublimate in exposed areas, as described, for example, in King etal., “Nanometer Patterning with Ice,” NANO Letters, Vol. 5, No. 6, pp.1157-60. (2005). Areas from which the ice has been removed can besubject to further processing, such as diffusion or metallization, whileother surface areas are protected by the ice layer.

In Gardener et al., “Ice-Assisted Electron Beam Lithography ofGraphene,” Nanotechnology 23 (2012) 185302, a thin layer of icecondensed on the surface acts as an EBIE precursor for etching agraphene layer underlying the ice. The electron beam is thought toinduce a reaction between the carbon in the graphene and the hydrogenand oxygen in the ice to form volatile carbon compounds that leave thesurface, removing the graphene.

In EBID, the dissociation product remains on the material, so theprocess can be carried out at low temperatures, at which the precursorgases adsorb more readily. In EBIE, however, the volatile reactionbyproducts must desorb from the surface, and low temperatures reduce thethermal desorption rate of the reaction products and are thereforeconsidered undesirable.

Bozso et al., “Electronic Excitation-Induced Surface Chemistry andElectron-Beam-Assisted Chemical Vapor Deposition,” Mat. Res. Soc. Symp.Proc., Vol. 158, pp. 201-209 (1990) describes a method of depositingsilicon, silicon nitride, silicon oxide, and silicon oxinitride filmsonto a silicon substrate using low energy EBID at a temperature ofapproximately 100 K (−173.degree. C.). The deposition method of Bozso isused to separate dissociation reactions caused by electrons from thosecaused by heat for more precise control over spatial growth and materialcomposition.

U.S. Pat. Publication No. 2012/0003394 of Mulders et al. for“Beam-Induced Deposition at Cryogenic Temperatures,” which is assignedto the assignee of the present invention, teaches choosing a precursorgas from a group of compounds having a melting point that is lower thanthe cryogenic temperature of the substrate and a sticking coefficientthat is between 0 and 1 at the desired cryogenic temperature. This isthought to result in the precursor gas reaching equilibrium betweenprecursor molecules adsorbed onto the substrate surface and precursorgas molecules desorbing from the substrate surface at the desiredcryogenic temperature before more than a small number of monolayers ofthe gas are formed. Suitable precursor gases include alkanes, alkenes,or alkynes, or the branch derivatives of those compounds.

It would be useful to find a method for EBIE that is not associated withthe problems described above.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide an improvedmethod for beam-induced etching.

A precursor gas is provided to a work piece surface that is maintainedat a temperature near the boiling point of the precursor gas, whichtemperature provides high precursor surface coverage withoutcondensation. The temperature also must be sufficiently high so as notto prevent desorption of reaction byproducts. In one preferredembodiment, the precursor is NF₃ and the work piece is silicon, and thework piece is cooled to well below room temperature, preferably to below−100° C.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a graph of the vapor pressure of NF₃ versus temperature.

FIG. 2 is a flowchart showing the steps of a preferred embodiment of thepresent invention.

FIG. 3 shows a graph of etch depth versus substrate temperature for a 30minute electron beam etch in an environment of NF₃.

FIG. 4 shows an image formed using gas cascade amplification with thesample in an environment of NF₃.

FIG. 5 shows schematically a typical dual beam system that can be usedto implement some embodiments of the invention.

FIG. 6 shows schematically a system having an environmental cell thatcan be used to implement some embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

EBIE allows removal of material from a substrate in a precise patternwith sub-micron accuracy. EBIE is most productive when the precursorcovers the surface and the reaction products readily desorb from thesurface to avoid redeposition and competition with precursor moleculesfor surface sites. While cooling the substrate is known to increasedeposition rates for EBID by improving coverage of the surface by theprecursor gas, it was thought that cooling the surface would decreasethe EBIE rate, or stop etching altogether, by reducing the desorptionrate of the reaction products. Applicants have found that, unexpectedly,the opposite occurs with certain gasses.

In some embodiments of the present invention, maximum precursor coverageon the surface is achieved by cooling the substrate to increase theresidence time of the precursor molecules on the surface. That is,precursor molecules adsorb onto the surface and remain, that is, do notthermally desorb, for a longer period of time when the substrate iscooled. However, the temperature must not be so low as to inhibitdesorption of the reaction products. Nor must the temperature be at orbelow the condensation point of the precursor gas. Condensed materialcollecting on the substrate surface interferes with desorption ofreaction products from that same surface; if desorption does notproceed, redeposition of material and competition for surface sites canoccur, preventing new precursor molecules from reaching the surface.Condensed material on the substrate surface also hinders imaging offeatures on that surface, making it hard to see when etching hasproceeded far enough. Moreover, some etch precursor gases are veryreactive and condensing them can result in violent reactions.

In some embodiments of the present invention, the substrate is cooled tomaximize precursor coverage by minimizing the adsorbate desorption rate.The minimum useful temperature is just above the condensation point ofthe precursor. However, the temperature must not be so low as to inhibitdesorption of the reaction product. Desorption occurs due to boththermal processes and to athermal processes, such electron stimulateddesorption, making it difficult to determine a minimum temperature atwhich the reaction products desorb theoretically. The minimumtemperature can be determined experimentally.

In some embodiments, the substrate is cooled to within 60° C. of theboiling point of the precursor gas, to within 40° C. of the boilingpoint, to within 20° C. of the boiling point, or to within 10° C. of theboiling point. In some embodiments, the substrate is cooled to belowroom temperature, to below −50° C., to below −100° C. or to below −125°C. In some embodiments, the temperature (in Kelvin scale) of the workpiece surface is maintained to within 20 percent of the temperature (inKelvin scale) of the boiling point of the precursor gas, within 10precent or within 5 percent. It is the temperature difference betweenthe substrate and the boiling point of the precursor, and not theabsolute temperatures, that is important. For some precursors with highboiling points, cooling the substrate may be unnecessary.

One precursor gas that applicants have found to work well on a cooledsubstrate is NF₃, a relatively inert gas having a much lower toxicitythan XeF₂. NF₃ does not spontaneously etch silicon, but upon activationby the electron beam, NF₃ reacts with a silicon substrate to form thereaction product SiF₄. NF₃ has similar oxidation properties to oxygen attemperatures less than 200° C., allowing it to be mixed with othergases, such as NH₃ and N₂O, during processing. NH₃ can remove residualcarbon effectively during the etch process and can prevent formation ofoxides that can act as an etch stop. In cases in which oxides mediateetching and are therefore desirable for the etch process, N₂O can bemixed with NF₃ to regenerate the oxide layer. Many useful gases can bemixed with NF₃ without requiring a complete evacuation of one gas beforeintroducing a second gas, that is, gas cycling, as required with morereactive gases. Because NF₃ is a gas at room temperature and can bestored in a pressurized container, there is negligible back diffusion ofa gas mix to contaminate the gas source. Unlike XeF₂, which reacts withmost gases including NH₃ and N₂O at room temperature, NF₃ does not reactwith these gases to produce unwanted products. Nor does NF₃ leavenon-volatile dissociation byproducts at the substrate surface, unlikeother sources such as CF₄, SiF₄ or BF₃.

NF₃ also has a geometry and ionization cross-section that makes it apromising candidate for environmental scanning electron microscope-typeimaging using gas cascade amplification imaging. Gas cascadeamplification imaging provides efficient charge stabilization of thework piece, as charged gas particles diffuse to the work piece toneutralize charged. This is particularly advantageous when etchinginsulating material such as SiO₂. Gas cascade imaging allows reliablemonitoring of the work piece while it is being etched.

Vapor pressures of gases are not well documented at conditions typicallyused in EBIE, that is, at pressures between about 10 Torr and 0.01 Torr.Data at just below atmospheric pressure is more readily available andcan be used as a guide. FIG. 1 shows that NF₃ remains a gas atapproximately −129° C. at 760 Torr. At the much lower pressures used inan SEM, condensation is expected to occur at a lower temperature.Applicants have found experimentally that EBIE of silicon using NF₃works well at a work piece temperature about −159° C. and a pressure ofabout 0.04 Torr.

The silicon is thought to react with the NF₃ to produce SiF₄ andnitrogen. The only readily available vapor pressure data for thereaction product SiF₄ is at 760 Torr with condensation occurring at −86°C. While one would suspect volatilization would not to occur during theprocess at −159° C. due to its higher boiling point of SiF₄ compared toNF₃, the etch process still proceeds, indicating that electronstimulated desorption may play a role in desorption.

In another embodiment, NF₃ is used as a precursor gas to etch siliconcarbide. This reaction is thought to be the same process as describedabove for silicon, but with the addition of carbon and therefore anadditional reaction product CF₄ as well as the reaction product SiF₄.The boiling point of CF₄ is −130° C. at 760 Torr, which is lower thanthe boiling point of SiF₄. One would expect that the limiting factor inregards to condensation of reaction products would be SiF₄. The workpiece surface would be maintained at the same temperature as forsilicon, as the precursor is the same and the desorption temperature islower for CF₄.

In another embodiment, diamond is etched using O₂ as a precursor gas.The carbon in the diamond combines with the oxygen to produce CO₂ and/orCO. Oxygen condensation occurs at about −215° C. at 4 Torr. CO₂condensation occurs at about −130° C. at 4 Torr. CO condenses at about−210° C. at 25 Torr. When using O₂ as a precursor gas to etch diamond orother carbon, one would cool down the work piece to preferably −130° C.and possibly lower to optimise the EBIE process.

FIG. 2 illustrates the steps of a preferred EBIE process. The processstarts in step 200, with loading a substrate into an apparatus, such asan SEM, a transmission electron microscope (TEM) or a laser system. Instep 202, a precursor gas is selected. Optionally in step 204, thesubstrate is cooled to a temperature determined by the properties of theprecursor gas chosen and the work piece material. Whether or not thesubstrate is cooled depends on the boiling temperature of the precursormolecules and the minimum temperature required to desorb reactionbyproducts. The substrate is cooled to a temperature close to theboiling point to maximize precursor coverage on the surface whilepreventing condensation and still allowing desorption of reactionproducts. In step 206, the selected precursor gas, which has a boilingpoint lower than the temperature of the substrate surface, is providedat the substrate surface. The pressure in the work piece chamber canrange from about 10⁻⁸ Torr when in an ultra-high vacuum system used witha gas injection system, to about 10 Torr when used in an environmentalscanning electron microscope-type system. In step 208, the substratesurface is irradiated in a pattern with a beam, such as an electronbeam, an ion beam, a cluster beam, or a neutral beam, in the presence ofthe precursor gas, so the precursor gas reacts in the presence of theparticle beam to remove material from the substrate surface. The workpiece is etched only where impacted by the beam, so a pattern can beetched having a resolution comparable to the spot size of the beam. Foran electron beam, the beam energy is preferably between 1 keV to 30 keV(SEM), and up to about 300 keV (TEM). The electron beam current ispreferably between 1 pA and 1 μA and the electron beam diameter or spotsize at the work piece is preferably between about 1 nm to 10 microns.An ion beam can also be used for IBIE. For example, a beam of galliumions from a liquid metal ion source can be used, as can a beam of argonor other ions from a plasma ion source, such as the Vion plasma ionsource from FEI Company. The ion energy is preferably between about 1keV to 50 keV, more preferably between 20 keV and 40 keV, and mostpreferable around 30 keV and a current of between about 1 pA to 1microamp. In step 210, the beam ceases to be directed to the substrate.

FIG. 3 shows experimental results for EBIE performed using an SEM havinga sample stage that is cooled by liquid nitrogen. Line 302 shows thedepth of hole produced by directing an electron beam toward silicon inan NF₃ environment for 30 minutes at various temperatures ranging fromroom temperature, 302 K, to 98 K. The graph shows that etch ratesincrease by about forty times by cooling the substrate. This increase inetch rate is believed to be a result of increasing the available NF₃ onthe surface through physisorption. The etch rate should scale withsurface coverage of NF₃ in the region irradiated by electrons, and thecoverage scales with reciprocal temperature. The prior art, on the otherhand, includes enhancing etching by heating the substrate to ensuredesorption of the reaction products, for example, using localizedheating by a pulsed laser beam. Thus, this embodiment is contrary toconventional thinking.

Based on their boiling points and vapour pressures, the followingprecursor gases are also likely to greatly benefit from cooling belowroom temperature: oxygen (O₂), nitrous oxide (N₂O), hydrogen (H₂),ammonia (NH₃), chlorine (Cl₂), hydrogen chloride (HCl). Substrateconstituents that cooling may be useful for include: boron (BF₃, B₂H₆)carbon (CO, CO₂, CF₄, CH₄), silicon (SiF₄, SiH₄), germanium (GeH₄,GeF₄), arsenic (AsH₃), phosphorus (PF₃, PH₃), tin (SnH₄), antimony(SbH₃), selenium (SeF₆), and sulphur (SF₆, SO₂). The molecules inbrackets are potential etch reaction products which are highly volatileand will likely desorb at the lower temperatures used to accelerateEBIE. The work pieces to which the above constituents are relevantinclude a range of compositions such as silicon, diamond, boron-dopeddiamond, germanium, silicon carbide, silicon nitride, silicon-germaniumalloys, boron nitride, boron phosphide & boron arsenide.

In the condensation method used in ice lithography, the precursor gas isapplied in the form of a “pulse” because the condensate (ice) thicknessgrows continuously while precursor gas is present in the vacuum chamber.In preferred embodiments of the invention, the precursor is deliveredcontinuously and the conditions are such as to prevent condensation. Theprecursor adsorbate thickness is self-limited by theadsorption/desorption kinetics, as in conventional room temperatureEBID. The condensation method is not preferred because it requiresmultiple additional steps and calibration of condensate thickness. Inthe case of environmental SEM beam chemistry, a gas phase precursor ispreferred so it can be used as an ESEM imaging gas. Imaging in “IceLithography” is limited due to the interaction of the electron beam withthe condensed H₂O layer. This greatly reduces the SEMs capability ofimaging substrate surface features.

Optionally, one can also use a cold trap above or near the sample toremove residual H₂O or hydrocarbons through condensation so as toincrease cleanliness of the chamber and therefore improve the processreproducibility/quality.

FIG. 4 is a photomicrograph showing an example of an image of insulatingsilica aerogel formed using NF₃ as the gas for gas cascadeamplification. Aerogels typically exhibit extreme, chronic chargingartefacts. Little optimization was needed to form this image,demonstrating how effective the gas is at stabilising charging. NF₃ isan excellent imaging medium, both for signal amplification and forcharge stabilization. It is much better than XeF₂, and similar in imagequality to NH₃, which has been previously shown to be better than H₂O.The aerogel in FIG. 4 was etched using an electron beam in the presenceof NF₃ mediated EBIE for about 5 to 10 minutes. The sample temperaturewas about −159° C. and the NF₃ pressure was about 0.04 Torr.

The present invention can be implemented in any beam-producing systemthat provides for providing a precursor gas at the work piece surface.For example, an electron beam system, an ion beam system, or a lasersystem can be used. FIG. 5 shows a typical dual beam system 510 suitablefor practicing the present invention, with a vertically mounted SEMcolumn and a focused ion beam (FIB) column mounted at an angle ofapproximately 52 degrees from the vertical. Suitable dual beam systemsare commercially available, for example, from FEI Company, Hillsboro,Oreg., the assignee of the present application. While an example ofsuitable hardware is provided below, the invention is not limited tobeing implemented in any particular type of hardware.

A scanning electron microscope 541, along with power supply and controlunit 545, is provided with the dual beam system 510. An electron beam543 is emitted from a cathode 552 by applying voltage between cathode552 and an anode 554. Electron beam 543 is focused to a fine spot bymeans of a condensing lens 556 and an objective lens 558. Electron beam543 is scanned two-dimensionally on the specimen by means of adeflection coil 560. Operation of condensing lens 556, objective lens558, and deflection coil 560 is controlled by power supply and controlunit 545.

Electron beam 543 can be focused onto substrate 522, which is on movableX-Y stage 525 within lower chamber 526. Stage 525 preferably isconnected to a cooler 527, such as a source of liquid nitrogen or aPeltier cooler, connected to the stage 525 by a thermal conduit 528.When the electrons in the electron beam strike substrate 522, secondaryelectrons are emitted. These secondary electrons are detected bysecondary electron detector 540 as discussed below. STEM detector 562,located beneath the TEM sample holder 524 and the stage 525, can collectelectrons that are transmitted through the sample mounted on the TEMsample holder as discussed below.

Dual beam system 510 also includes focused ion beam (FIB) system 511which comprises an evacuated chamber having an upper neck portion 512within which are located an ion source 514 and a focusing column 516including extractor electrodes and an electrostatic optical system. Theaxis of focusing column 516 is tilted 52 degrees from the axis of theelectron column. The ion column 512 includes an ion source 514, anextraction electrode 515, a focusing element 517, deflection elements520, and a focused ion beam 518. Ion beam 518 passes from ion source 514through column 516 and between electrostatic deflection meansschematically indicated at 520 toward substrate 522, which comprises,for example, a semiconductor device positioned on movable X-Y stage 525within lower chamber 526.

Stage 525 can also support one or more TEM sample holders 524 so that asample can be extracted from the semiconductor device and moved to a TEMsample holder. Stage 525 can preferably move in a horizontal plane (Xand Y axes) and vertically (Z axis). Stage 525 can also tiltapproximately sixty (60) degrees and rotate about the Z axis. In someembodiments, a separate TEM sample stage (not shown) can be used. Such aTEM sample stage will also preferably be moveable in the X, Y, and Zaxes. A door 561 is opened for inserting substrate 522 onto X-Y stage525 and also for servicing an internal gas supply reservoir, if one isused. The door is interlocked so that it cannot be opened if the systemis under vacuum.

An ion pump 568 is employed for evacuating neck portion 512. The chamber526 is evacuated with turbomolecular and mechanical pumping system 530under the control of vacuum controller 532. The vacuum system provideswithin chamber 526 a vacuum of between approximately 1×10⁻⁷ Torr and5×10⁻⁴ Torr. If an etch assisting, an etch retarding gas, or adeposition precursor gas is used, the chamber background pressure mayrise, typically to about 1×10⁻⁵ Torr.

The high voltage power supply provides an appropriate accelerationvoltage to electrodes in ion beam focusing column 516 for energizing andfocusing ion beam 518. When the ion beam strikes substrate 522, materialis sputtered, that is physically ejected, from the sample.Alternatively, ion beam 518 can decompose a precursor gas to deposit amaterial.

High voltage power supply 534 is connected to liquid metal ion source514 as well as to appropriate electrodes in ion beam focusing column 516for forming an approximately 1 keV to 60 keV ion beam 518 and directingthe same toward a sample. Deflection controller and amplifier 536,operated in accordance with a prescribed pattern provided by patterngenerator 538, is coupled to deflection plates 520 whereby ion beam 518may be controlled manually or automatically to trace out a correspondingpattern on the upper surface of substrate 522. In some systems thedeflection plates are placed before the final lens, as is well known inthe art. Beam blanking electrodes (not shown) within ion beam focusingcolumn 516 cause ion beam 518 to impact onto a blanking aperture (notshown) instead of substrate 522 when a blanking controller (not shown)applies a blanking voltage to the blanking electrode.

The liquid metal ion source 514 typically provides a metal ion beam ofgallium. The source typically is capable of being focused into a subone-tenth micrometer wide beam at substrate 522 for either modifying thesubstrate 522 by ion milling, enhanced etching, material deposition, orfor the purpose of imaging the substrate 522.

A charged particle detector 540, such as an Everhart Thornley ormulti-channel plate, used for detecting secondary ion or electronemission is connected to a video circuit 542 that supplies drive signalsto video monitor 544 and receiving deflection signals from controller519. The location of charged particle detector 540 within lower chamber526 can vary in different embodiments. For example, a charged particledetector 540 can be coaxial with the ion beam and include a hole forallowing the ion beam to pass. In other embodiments, secondary particlescan be collected through a final lens and then diverted off axis forcollection.

A micromanipulator 547, such as the AutoProbe 200™ from Omniprobe, Inc.,Dallas, Tex., or the Model MM3A from Kleindiek Nanotechnik, Reutlingen,Germany, can precisely move objects within the vacuum chamber.Micromanipulator 547 may comprise precision electric motors 548positioned outside the vacuum chamber to provide X, Y, Z, and thetacontrol of a portion 549 positioned within the vacuum chamber. Themicromanipulator 547 can be fitted with different end effectors formanipulating small objects. In the embodiments described herein, the endeffector is a thin probe 550.

A gas delivery system 546 extends into lower chamber 526 for introducingand directing a gaseous vapor toward substrate 522. U.S. Pat. No.5,851,413 to Casella et al. for “Gas Delivery Systems for Particle BeamProcessing,” assigned to the assignee of the present invention,describes a suitable gas delivery system 546. Another gas deliverysystem is described in U.S. Pat. No. 5,435,850 to Rasmussen for a “GasInjection System,” also assigned to the assignee of the presentinvention. For example, iodine can be delivered to enhance etching, or ametal organic compound can be delivered to deposit a metal.

A system controller 519 controls the operations of the various parts ofdual beam system 510. Through system controller 519, a user can causeion beam 518 or electron beam 543 to be scanned in a desired mannerthrough commands entered into a conventional user interface (not shown).Alternatively, system controller 519 may control dual beam system 510 inaccordance with programmed instructions. In some embodiments, dual beamsystem 510 incorporates image recognition software, such as softwarecommercially available from Cognex Corporation, Natick, Mass., toautomatically identify regions of interest, and then the system canmanually or automatically extract samples in accordance with theinvention. For example, the system could automatically locate similarfeatures on semiconductor wafers including multiple devices, and takesamples of those features on different (or the same) devices.

Embodiments of the invention can also be implemented using a highpressure SEM that uses gas cascade amplification. Such systems aredescribed, for example, in U.S. Pat. No. 4,785,182 to Mancuso et al.,entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.”

FIG. 6 shows a system using an environmental cell that can be filledwith the precursor gas comprising a particle-optical apparatus 600having a pinhole magnetic objective lens 602, a sample chamber 603having within it a sample cell 604 for maintaining a sample 606 at arelatively high pressure on a stage 608. A particle source (not shown)provides a primary electron beam 610 that passes through an upperpressure limiting aperture (PLA) 612 and a lower PLA 614 at the bottomof a cone 616. Cone 616 reduces the path of the electron beam 610through the gas in cell 604. Secondary electrons 620 emitted from thesample are detected by a secondary electron detector 622 built into cell604 and positioned to enable detection inside cell 604.

This system uses a detector 622 positioned apart from the specimenstage. The distance between detector 622 and the sample provides asufficient electron path for collisions between electrons leaving thesample and the gas to significantly amplify the electron current. Forexample, there are typically more than 300 electrons, more than 500electrons, or more than 1000 electrons reaching detector 622 for eachelectron leaving the sample. Detector 622 detects a current induced bythe flow of charge in the gas cascade to form an image. In otherembodiments, a photon detector can be used to detect photons emittedfrom the cascade to form an image. The photons are emitted by excitedions, fragments or neutrals, either in the gas or upon contact withsurfaces inside the sample cell or the sample chamber.

A gas input 624 and a gas output 626 regulate the flow rate and pressureof a process or imaging gas inside the sample cell 604. A source of aprecursor or imaging gas, such as NF₃, can be provided through gas input624. Gas output 626 is connected through a leak valve 628 to a roughingpump (not shown). A controlled leak through valve 628 and the relativelysmall volume of cell 604 compared to the volume of sample chamber 603provides for rapid switching between different processing gases, forexample, to switch between HPSEM beam chemistry mode and HPSEM imagingmode. Particle-optical apparatus 600 can function at relatively highpressure, that is, greater than 20 Torr (26 mbar). Particle-opticalapparatus 600 can also function at 50 Torr (65 mbar) or higher. In someembodiments, the pressure in the cell 604 is greater than 10 mTorr,while the pressure in the sample chamber 603 is less than 10 m Torr.

Secondary electron detector 622, which is shown in the form of a needlebut can also have different geometries such as a ring or a plate, iselectrically biased to preferably more than 100 V, more preferablygreater than 300 V, and most preferably about 500 volts to attractsecondary electrons, which collide with gas molecules between sample 606and secondary electron detector 622 to create an ionization cascade toamplify the secondary electron signal. The combination of cone 616 withthe configuration of secondary electron detector 622, which ispositioned outside of the cone, allows for a sufficient secondaryelectron path within the gas to provide adequate secondary electronsignal amplification, while maintaining a short primary electron paththrough the gas. The secondary electron path from the sample to thedetector is preferably greater than 2 mm. An optically transparentwindow 634 allows a user to observe the sample through an opticalmicroscope (not shown) using a lens 636 positioned between window 634and sample 606. The optical window 634 allows system 600 to provide awide field view, while still providing a short gas path length and a lowrate of gas leakage into the column, which improves resolution and imagesignal-to-noise ratio, and protects the column from corrosive gases.

Gas input 624 includes a valve arrangement 640 that allows for rapidswitching between multiple gas feeds, such as a one or more process gasfeeds 644 and an imaging gas feed 646. A duct 650 allows for evacuationof gases that pass through PLA 614, thereby helping to maintain a lowercolumn pressure above upper PLA 612. A stage 648, which can be the stagefrom a convention HPSEM or low pressure SEM into which cell 604 isplaced, allows the position of cell 604 to be adjusted so that PLA 614is aligned with the axis of electron beam 610, while stage 608 allowsmovement of the sample 606 within cell 604 so that a region of intereston the sample 606 can be positioned under the electron beam 610. A seal652, such as a Viton o-ring or a Teflon seal, preferably provides agas-tight seal between lens 602 and cell 604 to prevent gases from cell604 entering sample chamber 603. Seal 652 could also be a non-gas tightseal provided by a small gap that acts as a gas flow restriction betweenthe sub-chamber and sample chamber 603. Other systems that can use gascascade amplification are described, for example, in U.S. Pat. No.4,785,182 to Mancuso et al., entitled “Secondary Electron Detector forUse in a Gaseous Atmosphere” and U.S. Pat. No. 6,972,412 for“Particle-Optical Device and Detection Means,” to Scholtz et al., whichis assigned to the assignee of the present invention.

The invention is not limited to using a gas cascade detector in thecell. Conventional detectors, such as a gas luminescence detectors or athrough-the-lens style detector positioned about the PLA could also beused. In a through-the-lens style detector, voltages are applied to drawthe secondary particles back through the final lens, where they can bedetected by an on-axis or off axis collection system, such as amultichannel plate or a scintillator photomultiplier. FIG. 6 shows anoptional secondary electron deflector 654, such as a Wien filter, thatdefects secondary electrons away from the primary beam axis and intodetector 656, while passing the primary beam 610 without deviation.

While the description above uses the term “etch precursor” and“deposition precursor,” skilled persons will recognize that manyprecursors and precursor gas mixtures can either etch or deposit,depending on the gas flux and the beam density

Some embodiments of the invention provide a method of charged particlebeam etching, comprising cooling a work piece surface to below roomtemperature; exposing a substrate surface to a precursor gas, theprecursor gas being chosen from a group of compounds having a boilingpoint lower than the temperature of the substrate surface; andirradiating the substrate surface with a charged particle beam in thepresence of the precursor gas, the precursor gas reacting in thepresence of the particle beam to remove material from the substratesurface.

In some embodiments, exposing a substrate surface to a precursor gasincludes exposing the substrate surface the precursor gas comprises NF₃.In some embodiments in which the precursor gas comprises NF₃, the workpiece comprises silicon.

In some embodiments in which the precursor gas comprises NF₃, exposing asubstrate surface to a precursor gas includes exposing the substratesurface to a mixture of NF₃ and a gas that inhibits the growth of anoxide. In some embodiments in which the precursor gas comprises NF₃,exposing a substrate surface to a precursor gas includes exposing thesubstrate surface to a mixture of NF₃ and a carbon-etching gas. In someembodiments in which the precursor gas comprises NF₃, exposing asubstrate surface to a precursor gas includes exposing the substratesurface to a mixture of NF₃ and a gas that promotes the growth of anoxide. In some embodiments in which the precursor gas comprises NF₃,exposing a substrate surface to a precursor gas includes exposing thesubstrate surface to a mixture of NF₃ and NH₃. In some embodiments inwhich the precursor gas comprises NF₃, exposing a substrate surface to aprecursor gas includes exposing the substrate surface to a mixture ofNF₃ and N₂O.

In some embodiments, cooling a work piece surface to less than roomtemperature includes cooling the work piece surface to within 50° C. ofthe boiling point of the precursor. In some embodiments, cooling a workpiece surface to less than room temperature includes cooling the workpiece surface to within 20° C. of the boiling point of the precursor. Insome embodiments, cooling a work piece surface to less than roomtemperature includes cooling the work piece surface to less than minus100° C. In some embodiments, cooling a work piece surface to less thanroom temperature includes cooling the work piece surface to within 15percent of the boiling point on the Kelvin scale of the precursor gas.In some embodiments, the precursor gas comprises a compound havingmolecules that will adsorb to the substrate surface and dissociate inthe presence of the particle beam, thereby forming reactive fragmentswhich react with the substrate surface to form reaction products whichdesorb from the substrate surface.

In some embodiments, the precursor gas comprises oxygen, nitrous oxide,hydrogen, ammonia, chlorine, or hydrogen chloride. In some embodiments,the work piece comprises boron, carbon, silicon, germanium, arsenic,phosphorus, tin, antimony, selenium, or sulphur.

Some embodiment provide a method of forming an image of a work piece,comprising: directing an electron beam toward the work piece, the impactof the electron beam causing the emission of secondary electrons;accelerating the secondary electrons in a gas containing NF₃ to cause anionization cascade to amplify the secondary electron signal; detectingthe amplified secondary electron signal; and forming an image of thework piece using the amplified secondary electron signal.

In some embodiments, the method includes cooling the work piece to belowroom temperature.

Some embodiments provide a method of charged particle beam etching,comprising: selecting a precursor gas for beam-induced etching of a workpiece; maintaining a work piece surface at a temperature within 100° C.above the boiling point of the precursor gas; exposing a substratesurface to the precursor gas, the precursor gas being chosen from agroup of compounds having a boiling point lower than the temperature ofthe substrate surface; irradiating the substrate surface with a chargedparticle beam in the presence of the precursor gas, the precursor gasreacting in the presence of the particle beam to remove material fromthe substrate surface.

Some embodiments of the invention provide a charged particle beam systemfor processing a work piece, comprising: a charged particle source forproducing a primary beam of charged particles for irradiating saidsample; a focusing lens for focusing the charged particles onto a workpiece; an electrode for accelerating secondary electrons generated fromthe impact of the charged practice beam; and a container for enclosingNF₃, the electrode accelerating the secondary electrons though the NF₃to ionize the NF₃ in ionization cascade to amplify the number ofsecondary electrons for detection.

In some embodiments, the container for enclosing the NF₃ comprises asample vacuum chamber. In some embodiments, the container for enclosingthe NF₃ comprises an environmental cell. In some embodiments, thecontainer for enclosing the NF₃ comprises an ionization tube.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

We claim as follows:
 1. A method of forming an image of a work piece,comprising: flooding an environment surrounding the work piece with agas containing molecules of NF₃; cooling the work piece to below roomtemperature; directing an electron beam toward the work piece, theimpact of the electron beam causing the emission of secondary electronsfrom the work piece into the gas; causing a cascade ionization of thegas by accelerating the secondary electrons in the gas, wherein thecascade ionization of the gas amplifies the secondary electron signal;detecting the amplified secondary electron signal; and forming an imageof the work piece using the amplified secondary electron signal.
 2. Themethod of claim 1 in which the work piece comprises silicon.
 3. Themethod of claim 2, wherein the work piece comprises SiO₂.
 4. The methodof claim 1 in which the work piece comprises boron, carbon, silicon,germanium, arsenic, phosphorus, tin, antimony, selenium, or sulphur. 5.A method of processing a silicon-containing work piece, the methodcomprising: monitoring a work piece comprising silicon according to themethod of claim 1 while carrying out charged particle beam-inducedetching on the work piece, wherein the gas containing the NF₃ moleculesfunctions as an etch precursor gas for the charged particle beam-inducedetching.
 6. The method of claim 5, wherein the work piece comprisesSiO₂.
 7. A method of forming an image of a work piece, comprising:cooling the work piece to a temperature below 100° C. directing anelectron beam toward the work piece, the impact of the electron beamcausing the emission of secondary electrons; accelerating the secondaryelectrons in a gas containing NF₃ to cause an ionization cascade toamplify the secondary electron signal; detecting the amplified secondaryelectron signal; and forming an image of the work piece using theamplified secondary electron signal.
 8. The method of claim 7 in whichthe work piece comprises silicon.
 9. The method of claim 7 in which thework piece comprises boron, carbon, silicon, germanium, arsenic,phosphorus, tin, antimony, selenium, or sulphur.